For some time, master cylinders of the dual, tandem type have been used in automotive vehicles. Such cylinders have primary and secondary pressurizing chambers operated by respective primary and secondary pistons linearly disposed in a single main bore.
Vehicle brakes are commonly arranged in circuits, each circuit being connected to an outlet of a pressurizing chamber. A typical arrangement has front brakes operated by brake fluid from one pressurizing chamber and rear brakes operated by brake fluid from the other pressurizing chamber.
Known types of master cylinders require that the inlet, or replenishing port, of each pressurizing chamber from the master cylinder reservoir be disposed closer to the open end of the master cylinder housing than is the outlet.
Since this master cylinder has its open end recessed into a vacuum booster, no inlet connections can be readily made proximate this end. Accordingly, a long, obliquely angled, generally axially extending inlet passage may be provided in the master cylinder housing to communicate brake fluid from the reservoir to the pressurizing chamber disposed closest to the open end of the cylinder. This requires a relatively thick housing wall, adding weight and expensive machining to the system.
Such a system may also use a central control valve including a tappet slidably accommodated in a longitudinal bore of each piston and whose pedal-side end abuts a stationary bolt that extends transversely through the piston bore of the master cylinder and lifts the valve ball from its valve seat in the release position. To this end, the valve ball is held in a cage that encloses a rubber cushion or plug made of elastic material and that can be displaced in opposition to the force of a closure spring, all of which adds to the complexity and expense of the system.
Examples of such a system are shown in U.S. Pat. Nos. 4,979,426; 5,013;096; and 5,056,313. As shown in the latter of these, where the advantage of recessing the master cylinder in the vacuum booster is given up, the inlet passage of the primary piston can be located relatively near the open end of the cylinder.
A further example of the known prior art is shown in FIG. 1 of the drawing, which is a sectional representation of a typical, standard, prior art master cylinder. The master cylinder is shown including an elongate housing having a main bore. The housing also has primary and secondary inlet passages that are connected to a brake fluid reservoir.
Primary and secondary pistons are disposed within the main bore, defining a primary pressurizing chamber between the primary and secondary pistons and a secondary pressurizing chamber between the secondary piston and a closed end of the housing. The pistons are slidable between unstroked and stroked positions. Communication is maintained between the reservoir and each secondary pressurizing chamber through secondary vent ports during a compensation cycle when the pistons are returning to their unstroked position.
In operation, as the pistons begin to move toward the closed end of the housing, at least one of the seals on each of the primary and secondary pistons slide over the orifices of the primary and secondary vent ports of the standard master cylinder.
To reduce seal wear and increase seal longevity, the diameter of the primary and secondary vent ports proximate their intersection with the main bore are necessarily small compared to those of the primary and secondary inlet passages. The small orifices can result in increased back pressure, however, especially when brake fluid is cold.
While the prior apparatuses function with a certain degree of efficiency, none disclose the advantages of the improved master cylinder of the present invention as is hereinafter more fully described.